1. Field of the Invention
The present invention relates to a method for operating a hybrid vehicle, in which an idle first drive unit is started while the hybrid vehicle is being driven, by transferring a torque from an operating second drive unit to the first drive unit, and a device for implementing this method.
2. Description of Related Art
Vehicles having a hybrid drive structure usually have an internal combustion engine as the first drive unit and an electric motor or a hydraulic motor as the second drive unit. Other additional drive units are also possible. For example, the torque may be applied by the drive units during the starting operation of the hybrid vehicle. All-electric or hydraulic driving is also possible. It may be necessary to start the internal combustion engine during electric or hydraulic driving, for example, if the driver requires more power than the electric motor or the hydraulic motor is able to supply or if the energy content of an energy storage mechanism drops too significantly.
A separating clutch, which couples the internal combustion engine for the start in a slipping state, has inaccuracies. In addition, the required time characteristic of the starting torque or the starting power is not precisely determinable due to varying friction and compression conditions of the internal combustion engine. Both prevent accurate compensation of the starting torque or the starting power by the electric motor or the hydraulic motor. The uncompensated portion acts as a disturbance on the drive train that causes rotational vibrations and reduces driving comfort.
Published German patent application document DE 10 2006 047 655 A1 describes a method for operating a parallel hybrid drive, which has an electric machine and an internal combustion engine. When the vehicle is being driven, the internal combustion engine is started with the aid of the electric machine by engaging the separating clutch. During regulation in which an operating variable of the parallel hybrid drive is detected and compared with a corresponding model operating variable of a model of the parallel hybrid drive, a deviation of the electric machine resulting from the comparison is at least partially compensated.